Comparing the nitrogen and phosphorus requirements of canola and wheat for grain yield and quality

Canola (oilseed rape, Brassica napus L.) is now grown in rotation with spring wheat (Triticum aestivum L.) on the predominantly sandy soils of south-western Australia. For both crop species, fertiliser nitrogen (N) and phosphorus (P) need to be applied for profitable grain production. The fertiliser N requirements have been determined separately for canola or wheat when adequate P was applied. By contrast, the fertiliser P requirements of the 2 species have been compared in the same experiment when adequate N was applied and showed that canola consistently required similar to 25-60% less P than wheat to produce 90% of the maximum grain yield. We report results of a field experiment conducted at 7 sites from 2000 to 2003 in the region to compare grain yield responses of canola and wheat to application of N and P in the same experiment. Four levels of N (0-138 kg N/ha as urea [46% N]) and 6 levels of P (0-40 kg P/ha as superphosphate [9.1% P]) were applied. Significant grain yield responses to applied N and P occurred for both crop species at all sites of the experiment, and the N x P interaction for grain production was always significant. To produce 90% of the maximum grain yield, canola required similar to 40% more N (range 16-75%) than wheat, and similar to 25% less P(range 12-43%) than wheat. For both crop species at 7 sites, applying increasing levels of N had no significant effect on the level of P required for 90% of maximum grain yield, although at 1 site the level of P required to achieve the target yield for both crop species when no N was applied (nil-N treatment) was significantly lower than for the other 3 treatments treated with N. For both crop species at all 7 sites, applying increasing levels of P increased the level of N required for 90% of the maximum grain yield. Fertiliser P had no significant effect on protein concentration in canola and wheat grain, and oil concentration in canola grain. As found in previous studies, application of increasing levels of N decreased oil concentration while increasing protein concentration in canola grain, and increased protein concentration in wheat grain. The N x P interaction was not significant for protein or oil concentration in grain. Protein concentrations in canola grain were about double those found in wheat grain.